I. Field of the Invention
This invention relates to molecular forms of carbon known as fullerenes wherein the fullerene cage encloses one or more metal atoms and to the process for making the fullerenes. A fullerene is a third form of pure carbon and is different from graphite and diamond, the only two forms known before 1985, see "Fullerenes," Curl, R. F. and Smalley, R. E., Scientific American, October, 1991, pp. 54-63, incorporated herein by reference, and references cited therein.
A fullerene structure is characterized in that each carbon atom is bonded to three other carbon atoms. The carbon atoms so joined curve around to form a molecule with a cage-like structure and aromatic properties. A fullerene molecule with 60 carbon atoms, referred to as "buckminsterfullerene," resembles the familiar shape of a soccer ball. A structural diagram representing C.sub.60 is shown in FIG. 1. Fullerenes may contain even numbers of carbon atoms totalling from 20 to 500 or more. FIG. 2 shows the structure of a C.sub.70 fullerene and FIG. 3 shows the structure of a C.sub.84 fullerene. Fullerenes are not necessarily spherical. They may take the form of long tubular structures with hemispherical caps at each end of the tube. Hyperfullerene structures also exist wherein one structure is contained within a second larger structure. For generally spherical molecular structures, these hyperfullerenes resemble an onion layered structure. Tubular structures within larger structures are also possible. Fullerenes are more fully described in the literature cited above.
II. Description of the Prior Art
The molecular structure for buckminsterfullerene was first identified in 1985, see NATURE, C.sub.60. "Buckminsterfullerene", Kroto, H. W., Heath, J. R., O'Brien, S. C., Curl, R. F. and Smalley, R. E., Vol. 318, No. 6042, pp. 162-163, Nov. 14, 1985. The process described therein for making fullerenes involves vaporizing the carbon from a rotating solid disk of graphite into a high-density helium flow using a focused pulsed laser. That process did not utilize a temperature controlled zone for the growth and annealing of fullerene molecules from the carbon vapor formed by the laser blast. Only microscopic quantities of fullerenes were produced by this process.
Another method of making fullerenes was described in THE JOURNAL OF PHYSICAL CHEMISTRY, "Characterization of the Soluble All-Carbon Molecules C.sub.60 and C.sub.70," Ajie et. al, Vol. 94, Nov. 24, 1990, pp. 8630-8633. The fullerenes are described as being formed when a carbon rod is evaporated by resistive heating under a partial helium atmosphere. The resistive heating of the carbon rod is said to cause the rod to emit a faint gray-white plume. Soot-like material comprising fullerenes is said to collect on glass shields that surround the carbon rod.
Another method of forming fullerenes in greater amounts is disclosed in U.S. Pat. No. 5,227,038 filed Oct. 4, 1991 wherein carbon is vaporized in an electrical arc and the carbon vapor condenses into fullerenes. U.S. Pat. No. 5,227,038 is incorporated herein by reference.
One disadvantage of the prior art is the low yield of fullerenes containing metal atoms. Another disadvantage of the prior art is the inability to produce fullerenes containing two or more metal atoms inside the fullerene cage. These and other disadvantages of the prior art are overcome by the present invention which provides a method of producing fullerenes in macroscopic quantities, some containing metal atoms inside the fullerene cage. Atoms of elements other than carbon may also be incorporated in the fullerene cage network. The present invention also provides a composition comprising fullerenes with two or more metal atoms inside the fullerene cage and a method for making such a composition.